The semiconductor industry has experienced rapid growth due to improvements in the integration density of a variety of electronic components (e.g., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from shrinking the semiconductor process node (e.g., shrink the process node towards the sub-20 nm node). As semiconductor devices are scaled down, new techniques are needed to maintain the electronic components' performance from one generation to the next.
As semiconductor technologies evolve, metal oxide semiconductor (MOS) transistors have been widely used in today's integrated circuits. MOS transistors are voltage controlled device. When a control voltage is applied to the gate a MOS transistor and the control voltage is greater than the threshold of the MOS transistor, a conductive channel is established between the drain and the source of the MOS transistor. As a result, a current flows between the drain and the source of the MOS transistor. On the other hand, when the control voltage is less than the threshold of the MOS transistor, the MOS transistor is turned off accordingly.
MOS transistors may include two major categories. One is n-channel MOS transistors; the other is p-channel MOS transistors. According to the structure difference, MOS transistors can be further divided into three sub-categories, planar MOS transistors, lateral double diffused MOS transistors and vertical double diffused MOS transistors.
As semiconductor technologies further advance, silicon-on-insulator (SOI) based junction-less MOS transistors have emerged as an alternative technique to simplify the manufacturing process of MOS transistors. In a junction-less MOS transistor, a channel region of the junction-less MOS transistor is of the same polarity and the same doping concentration as the drain/source regions of the junction-less MOS transistor.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.